1. Field of the Invention
This invention relates to monitoring the cleaning and drying processes during the manufacture of ICs, MEMS and other micro-devices and more specifically to a micro sensor for high aspect ratio micro features in dielectric films oriented perpendicular to the fluid-solid interface.
2. Description of the Related Art
A major challenge in manufacturing of the micro and nano devices is the cleaning and drying of very small void features (“micro features”), particularly those with large aspect ratios. These micro features are fabricated in various processing steps and can be very small voids such as gaps, holes, vias or trenches that are intentionally etched. The micro features can also be pores (voids) in a deposited dielectric material. Cleaning and drying occur repeatedly during the processing chain and are, responsible for a significant part of the total processing time and for the consumption of much of the water, chemicals and energy.
In semiconductor manufacturing, trenches and vias are fabricated both in the device level and in the interconnect level. Most of these features have high aspect ratios with submicron openings that are oriented perpendicular to the fluid-solid interface of the device to the cleaning fluid and because of their high aspect ratio and very small width are very difficult to clean and dry. In Integrated Circuits, MEMS and other micro device manufacturing, well controlled cleaning and drying are essential to avoid deformation of layers and improper adhesion of moving parts. Improper cleaning and drying would have a significant effect on manufacturing yield and device performance and reliability in both semiconductor and MEMS fabrication. Over-cleaning, over-rinsing or over-drying results in excessive use of chemicals, water and energy and also increases cycle time and potentially causes yield loss. Therefore, there is a strong economic and environmental incentive to use a process that is “just good enough”.
The fine structures left behind after processes such as etching, deposition, and patterning, need to be cleaned and the reaction by-products need to be removed often down to trace levels. This usually involves three steps: 1) application of a cleaning solution; 2) rinsing and/or purging using ultra pure water or other rinsing solutions; and 3) drying by removing and purging the traces of any solvents used during rinsing. Due to the undesirable surface tension associated with aqueous chemicals and non-wetting nature of most future dielectrics, industry is pursing the development of processes based on supercritical fluids such as supercritical carbon dioxide for cleaning and pattern development. Measurement of cleanliness under these processing conditions is very critical.
Cleaning, rinsing, and subsequent drying processes are often performed and controlled almost “blindly” and based on trial and error or past experience. The way these processes are monitored and controlled presently is based on ex-situ testing of wafer, chips, or structures. Within the process tool, fixed recipes are provided by tools and process suppliers. Run-by-run adjustments or control are based on external and delayed information on product performance or product yields. The key reason for this inefficient and costly approach is that no sensors or techniques are available to measure the cleanliness and monitor the removal of impurities from micro features—to measure cleanliness where it actually counts. The sensors that are currently available are used in the fabs to monitor the conditions of fluid inside the process vessels and tanks, but far away from the inside of micro features (that is what needs to be monitored; it is also the bottleneck of cleaning and drying). The present monitoring techniques and devices do not provide realistic and accurate information on the cleanliness and condition of micro features.
Industry currently works around this problem while waiting for a solution; the process condition and cleaning and drying are often set with very large factors of safety (over-cleaning and over-rinsing). Large quantities of water and other chemicals are used (much more than what is really needed). This results in wasted chemicals, increased process time, lowered throughput, increased cost, and it causes reliability issues because of lack of process control.
K. Romero et al “In-situ analysis of wafer surface and deep trench rinse,” Cleaning Technology in Semiconductor Device Manufacturing VI, The Electrochemical Society, 2000 propose a trench device for monitoring the process in-situ. As shown in FIG. 1a, a trench device 10 comprises a pair of conducting electrodes (Poly-Si) 12 and 13 sandwiched between dielectric (SiO2) layers 16 and 17 on opposite sides of a trench 14 on a substrate 18. Trench 14 is oriented perpendicular to the fluid-solid interface 19 of the device. An impedance analyzer 20 applies a measurement signal (voltage and current) 21 to the electrodes, which carry the measurement signal to the trench. The impedance analyzer measures the impedance between its two terminals (the impedance consists of the ratio of the voltage and current and the phase difference between the voltage and current).
The electrical equivalent circuit diagram of trench device 10 is presented in FIG. 1b. The sensor is configured to measure the solution resistance Rsol'n 22, which is dependent on the ionic concentration of impurities in the fluid 23 inside the trench 14. At solid-solution interfaces, an interface double layer forms because charges in the solution that are mobile (ions) respond to the presence of fixed charges on the solid. The interface double layer is responsible for a capacitance Cdl 24 between the electrode and the solution, which forms an impedance Zdl=1/jωCdl where ω is the measurement signal radial frequency. The impedance is in series with Rsol'n.
Since the sensor measures the solution resistance through two series capacitors, the measurement must be performed using an ac signal. If the series impedance Zdl is much larger than Rsol'n, (i.e. if Cdl is small and/or the measurement radial frequency ω is small so that Rsol'n<<1/jωCdl), then the sensor's impedance output is dominated by Cdl and the solution resistance Rsol'n can not be effectively measured.
The electrodes also form parasitic capacitances with other conductors in their neighborhood. The total parasitic capacitance is primarily between the electrodes 12 and 13 and the substrate 18 represented by capacitors Csubstrate 26 and 27. There can also be significant capacitance between the electrode and the fluid above the electrode, and this is represented by the capacitors Cfluid 28 and 29. The parasitic capacitances form parasitic shunt circuits across the solution resistance. These shunt circuits are in parallel with the solution resistance and therefore allow the measurement signal 21 to bypass the solution resistance. If the shunt circuit impedance is significantly lower than the solution resistance, then the sensor's impedance output is dominated by the parasitic capacitances and the solution resistance can not be effectively measured.
For the sensor to be useful as a monitor of the fluid in the micro feature, the total parasitic capacitance must be sufficiently small to allow an electrical measurement of the total impedance between the electrodes to resolve Rsol'n and/or Cdl. If the parasitic capacitance dominates the total electrical response, then the circuit will not have a good signal to noise ratio and the sensor will not be very sensitive. In the paper by Romero et al., the parasitic capacitance was found to dominate the solution resistance. At the parasitic capacitance measured (88 pF), the equivalent circuit calculation predicts no discernable impedance variation between highest and lowest trench resistances. The full ionic concentration range was not experimentally resolvable in comparison to electronic noise. Despite the difficulties encountered, a trench resistivity device still holds promise in resolving the process chemical evolution out of a sub-micron trench.